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CHICAGO -- The father of a 3-year-old boy has been jailed and charged with felony child endangerment after his 6-year-old son accidentally shot and killed his younger brother, Chicago police said.

The boys were playing "cops and robbers" when the gun went off, according to authorities. It happened while their dad, Michael Santiago, 25, was at work, their mom was at the store and the boys' grandfather was watching them.

Chicago police said the 6-year-old found a loaded gun his dad had stashed in the kitchen, wrapped in pajama pants.

"Michael Santiago was arrested after and charged after he knowingly stored a loaded firearm on top of a refrigerator with access to his minor children in their home," police said.

Grandfather Israel LaSalle, who lives in the unit above the family, sent the boys downstairs to get something to clean up after the dog with and heard a gun go off a short time later, according to CNN affiliate WLS.

"I heard a pop, you know, like somebody had shot, so I opened the door and I look out and he was running up the stairs and he told me somebody shot his brother," LaSalle told the affiliate.

The 3-year-old victim was taken by a family member to Norwegian American Hospital and later transferred to Mt. Sinai Hospital where he died.

"You know how kids are, they get into everything," he said. "There is not a safe place where you can put a gun where a kid can't reach it or find it."

Santiago is being held on $75,000 bond.

Investigators say he is a former gang member who illegally purchased the gun for protection after testifying against another gang member in a murder trial.